On Thursday, a tiny US robot spacecraft will make a dramatic encounter in deep space. The probe will sweep past a comet known as Hartley 2 and take detailed measurements and photographs.

The results that will be sent back by Nasa's Epoxi spaceship are keenly awaited – previously scientists have found that the more they learn about comets the more baffling and mysterious they appear to be. They hope that this time round, some of those mysteries may be resolved.

In recent years, space scientists have discovered one comet that contains minerals created at extremely high temperatures, another that belches gases containing cyanides, one that has a surface fluffier than snow and another that has a firm surface peppered with craters.

"Comets were once thought to be roughly similar in shape and structure," said Dr Mark Bailey, of Armagh Observatory, in Northern Ireland. "But the more probes that we fly past them, the more differences we find. Goodness knows what we are going to find this time at Hartley 2."

Comets are formed from space rubble – dust, ice and small pieces of rock – left over from the making of our solar system billions of years ago and vary in size from a few hundred metres in diameter to tens of kilometres. Most orbit the Sun at the very edge of the solar system and only occasionally sweep close to Earth. When they do, radiation from the Sun causes their surfaces to heat up and release gas and dust. This material forms the comet's tail and gives these visitors to the night sky their distinctive appearance.

Scientists believe that water and complex organic materials detected on comets may have played a key role in the evolution of life on Earth. Hence their interest in studying them close up.

To do this, four space probes – one European and three American – have been sent on missions to rendezvous with comets. The first mission took place in 1986 when Europe's Giotto probe passed close to Halley's comet and sent back photographs of a dark surface from which jets of glowing material were emerging. Fifteen years later, Nasa's Deep Space 1 probe swooped close to Borrelly's comet and revealed a much smaller object with a more varied surface that appeared to be warm and dry.

Then in 2004, Nasa's Stardust probe flew past the comet Wild 2 and returned to Earth with particles it had collected from its tail. These were found to include crystalline silicates that are only formed at high temperatures. For an astronomical body that is generally thought to orbit in deep, cold space, these compounds are a puzzle, scientists admit.

And finally, a comet called Tempel 1 was visited by Nasa's Deep Impact probe in 2005. The spacecraft fired a copper projectile into the comet to study the debris thrown up. It also carried out a detailed photographic survey of the comet which showed it was littered with craters that must have been caused by smaller objects striking it in the past.

Given that a comet's surface is supposed to change every time it orbits close to the Sun, as radiation drives material from it, the existence of these old craters is also a puzzle. Only an unchanging surface could preserve craters, scientists have pointed out. "It seems like every time we go to a new comet, we discover new phenomena," said Dr Lori Feaga, an astronomer at the University of Maryland, and a member of the Epoxi science team.

Now astronomers have a new comet to target, though they are using an old probe to study it. After Deep Impact swept past its first target, the mission was given a new name, Epoxi, and sent on an orbit that will bring it to within 430 miles of Harley 2 this week. Already the mission has revealed fresh surprises. Last month, its instruments showed that the comet was emitting a toxic gas called cyanogen whose output increased fivefold over an eight-day period before slowly decreasing again. Mission scientists are still arguing over interpretations of this data.

"Comets have more than one source," said Bailey. "Some are found at the very edge of the solar system in a great band known as the Oort cloud. Others are found in a belt closer to the Sun known as the Kuiper belt. And then others can be created by asteroid collisions or some other violent event. So we should perhaps not be too surprised if they are turning out to be very different from each other."

Many of these new mysteries may be solved in 2014 when the European Space Agency's Rosetta probe reaches the comet Churyumov-Gerasimenko in 2014 and follows it, for more than a year, as it passes near the Sun. On the other hand, added Bailey, the mission may simply throw up even more puzzles.